Straighter boosts for gliders

[brewster_rockit]

I'm sure this has been asked before, but here goes:

In my experience, my boost gliders have tended to have an S-shaped boost. The rocket tends to pitch nose-down during the powered portion of the flight (thrust line higher than the CG,) then pitch up as the aerodynamic forces on the glider become dominant during coast to apogee. It's best to pick short delays to minimize the latter, although that's not always possible. Plenty of gliders loop during the coast phase; my Estes Nighthawk clone tends to loop onto its back and coast horizontally until ejection on a B4-2. For the experienced glider builders, what tricks and tips help to ensure the straightest possible boost? Perhaps pylon height and location relative to the glider GC, launch lug placement, maybe ballast in the pod? Is there a good rule of thumb for where the boost CG should be relative to the glide CG? Thanks!

 

[Alan15578]

In general, both boost gliders and rocket gliders need to be flight trimmed, especially new designs. This includes not just glide trimming, but boost trimming as well. This is acomplex subject, but yes, some people like far forward boost CGs,

 

[brewster_rockit]

It's certainly worth taping the pod to the glider and chucking it to trim for a straighter boost (even better, drop it vertically from an upper-story window.) I'm just thinking about some rules of thumb to design and build the pod "smartly" to minimize the amount of ballast that would need to be used to trim for good boost.

 

[Amsterdam]

It's certainly worth taping the pod to the glider and chucking it to trim for a straighter boost (even better, drop it vertically from an upper-story window.) I'm just thinking about some rules of thumb to design and build the pod "smartly" to minimize the amount of ballast that would need to be used to trim for good boost.

in theory the farther forward you put your ballast the less weight you need, and instead of dropping a BG/RG in boost mode you should opt for a swing test, like you would a traditional LP rocket.

 

[Alan15578]

It's certainly worth taping the pod to the glider and chucking it to trim for a straighter boost (even better, drop it vertically from an upper-story window.) I'm just thinking about some rules of thumb to design and build the pod "smartly" to minimize the amount of ballast that would need to be used to trim for good boost.

I think you need to burn motors and do real testing, There are lots or things you can adjust. However, you can tape the pop pod to the glider to avoid loss of the glider, although transition to glide can often be tuned up as well.

The classic S boost is nice. It is much better than actual looping and pranging.
Symmetry is a wonderful thing. If you want straight, high boosts, fly electable flexwing gliders.
I have not flown an Estes Nighthawk, so I cannot help you with that one.

 

[UhClem]

It has been a while but I vaguely remember a NARCON presentation on glider trimming. As I recall the basic idea was to trim the glider so it flew near stall and add a little washout (upward deflection) to one wing.

Then during glide the wing without the washout would stall before the other. This would cause it to turn in those nice lazy circles we like.

During boost the washout would cause a slow roll which would help with any tendency to pitch up or down.

 

[brewster_rockit]

I recall seeing from a prominent glider flyer on the forum that inducing a slow roll on boost was the best way to get a straight boost. He suggested mounting the pod at a slight angle to the centerline of the glider. Might try that on the next pod I build.

When you say "washout" are you suggesting a spanwise twist, so that the tip airfoil is rotated nose-down relative to the root airfoil?

 

[Buzzard]

FlyBaby-B-BG-Russell.pdf (nar.org)

A long boost pod will help move the Center of Gravity foward, which helps with a staight boost. A slow roll will also help. The glider design above uses one wing tip a half inch higher than the other to make it turn. Washout will also do that and can induce a roll during boost.

Chas

 

[Aeronerd]

I'm sure this has been asked before, but here goes:

In my experience, my boost gliders have tended to have an S-shaped boost. The rocket tends to pitch nose-down during the powered portion of the flight (thrust line higher than the CG,) then pitch up as the aerodynamic forces on the glider become dominant during coast to apogee. It's best to pick short delays to minimize the latter, although that's not always possible. Plenty of gliders loop during the coast phase; my Estes Nighthawk clone tends to loop onto its back and coast horizontally until ejection on a B4-2. For the experienced glider builders, what tricks and tips help to ensure the straightest possible boost? Perhaps pylon height and location relative to the glider GC, launch lug placement, maybe ballast in the pod? Is there a good rule of thumb for where the boost CG should be relative to the glide CG? Thanks!

If I recall correctly the Nighthawk's boost pod had canard flaps that could be adjusted to help c ompensate and give you a more vettical launch.

 

[brewster_rockit]

I've been messing around with the Nighthawk canards to see if I can get enough pitch-down to compensate for the rocket's propensity to pitch up and over. Now I'm thinking that differential pitch on the canards might be a better idea, if it'll induce a roll during boost.

 

[Alan15578]

I've been messing around with the Nighthawk canards to see if I can get enough pitch-down to compensate for the rocket's propensity to pitch up and over. Now I'm thinking that differential pitch on the canards might be a better idea, if it'll induce a roll during boost.

I still have no experience with the Nighthawk. A rolling boost can be helpful, especially with long burning motors. However, you should minimize your boost pitch problem first. Adding roll to bad pitch trim will likely just compound your problems.